[Mesa-dev] [PATCH] i965/fs: Fix Sandybridge regressions from SEL optimization.

[Kenneth Graunke]

On 08/13/2013 05:49 PM, Ian Romanick wrote:
> On 08/13/2013 04:47 PM, Kenneth Graunke wrote:
>> Sandybridge is the only platform that supports an IF instruction
>> with an embedded comparison.  In this case, we need to emit a CMP
>> to go along with the SEL.
>>
>> Fixes regressions in Piglit's glsl-fs-atan-3, fs-unpackHalf2x16,
>> fs-faceforward-float-float-float, isinf-and-isnan fs_basic, and
>> isinf-and-isnan fs_fbo.
>>
>> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
>> ---
>>   src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 17 +++++++++++++----
>>   1 file changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
>> b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
>> index a36c248..984b08a 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
>> @@ -1911,10 +1911,19 @@ fs_visitor::try_replace_with_sel()
>>            emit(MOV(src0, then_mov->src[0]));
>>         }
>>
>> -      fs_inst *sel = emit(BRW_OPCODE_SEL, then_mov->dst, src0,
>> else_mov->src[0]);
>> -      sel->predicate = if_inst->predicate;
>> -      sel->predicate_inverse = if_inst->predicate_inverse;
>> -      sel->conditional_mod = if_inst->conditional_mod;
>> +      fs_inst *sel;
>> +      if (if_inst->conditional_mod) {
>> +         /* Sandybridge-specific IF with embedded comparison */
>
> This doesn't appear to be SNB-specific code.  Can you explain this?  Is
> if_inst->conditional_mod only set on SNB?  I really need to learn more
> about the back end...

Normally, control flow looks like:

cmp.l.f0(8)  null   g5<8,8,1>F    0F
(+f0) if(8)

For Sandybridge, the hardware designers extended IF to support built-in 
comparisons, so you can simply do:

if.l(8)    g5<8,8,1>F    0F

They immediately dropped this with Ivybridge; it's not been present on 
any other platform.

fs_inst::conditional_mod represents that conditional modifier (always = 
0, never, less, equal, lequal, greater, notequal, gequal).